An induction heating (IH) type of fixing apparatus generates an eddy current in a heat-producing element through the action of a magnetic field generated by a magnetic field generation unit, and heat-fixes an unfixed image on a recording medium such as transfer paper or an OHP sheet through Joule heating of the heat-producing element by means of the eddy current. An advantage of this induction heating type of fixing apparatus compared with a heat roller type of fixing apparatus that uses a halogen lamp as a heat source is that heat production efficiency is higher and the fixing speed can be increased.
With this kind of fixing apparatus, startup responsiveness when the heat-producing element is heated can be markedly improved by using a heat-producing roller comprising a thin sleeve or a heat-producing belt comprising an endless belt as the heat-producing element, and making the thermal capacity of the heat-producing element low.
With this kind of fixing apparatus, if heat-fixing is performed of small-size paper with a paper width smaller than the heating width of the heat-producing element when a paper passage area of large-size paper of the heat-producing element has been heated, the temperature of the paper passage area of small-size paper of the heat-producing element falls after that heat-fixing. This is because heat of the paper passage area of the heat-producing element is absorbed by the small-size paper passed through.
Thus, in this kind of fixing apparatus, in order to suppress the occurrence of fixing defects due to this fall in temperature of the heat-producing element due to the passage of small-size paper, the heat-producing element is heated with heating power greater than the normal heating power when paper is not passed through, and the temperature of a paper passage area of small-size paper of the heat-producing element is maintained at a predetermined fixing temperature.
Therefore, with this kind of fixing apparatus, when a paper passage area of small-size paper of the heat-producing element is heated with high heating power, a paper non-passage area of the heat-producing element is heated due to the effect of this heating. As a result, with this fixing apparatus, a paper non-passage area of the heat-producing element experiences an excessive rise in temperature and temperature distribution in the width direction of the heat-producing element becomes uneven, and when large-size paper is passed through, glossiness abnormalities and hot offset of a fixed image tend to occur. The temperature difference between a paper passage area and paper non-passage area of the heat-producing element due to this kind of excessive rise in temperature of a paper non-passage area of the heat-producing element increases with the quantity of small-size paper of the same width passed through continuously.
A known technology for eliminating the above-described excessive rise in temperature of a paper non-passage area is one whereby, of the magnetic flux generated by an exciting apparatus that performs induction heating of the heat-producing element, only magnetic flux that acts on a paper non-passage area of the heat-producing element is absorbed by a magnetic flux absorption member capable of moving in the paper passage area width direction of the heat-producing element, and heat production of a paper non-passage area of the heat-producing element is suppressed (see, for example, Patent Document 1 and so forth).
Another known technology for eliminating the above-described excessive rise in temperature of the paper non-passage area is one whereby, based on an image forming condition such as the recording medium size, alternation is performed between rotational cooling that cools by idling a heating roller serving as the heat-producing element and a pressure roller, and static cooling whereby cooling is performed with rotation of the heating roller and pressure roller stopped (see, for example, Patent Document 2 and so forth).
FIG. 1 is a schematic oblique drawing of a sample implementation of a fixing apparatus disclosed in Patent Document 1. As shown in FIG. 1, this fixing apparatus is provided with a coil assembly 10, a metal sleeve 11, a holder 12, a pressure roller 13, a magnetic flux masking shield 31, a displacement mechanism 40, and so forth.
In FIG. 1, coil assembly 10 generates a high-frequency magnetic field. Metal sleeve 11 is heated by an induction current induced by an induction coil 18 of coil assembly 10, and rotates in the direction of transportation of recording material 14. Coil assembly 10 is supported inside holder 12. Holder 12 is fixed to a fixing unit frame (not shown) and does not rotate. Pressure roller 13 rotates in the direction of transportation of recording material 14 while pressing against metal sleeve 11 and forming a nip area. By having recording material 14 gripped and transported by means of this nip area, an unfixed image on recording material 14 is heat-fixed to recording material 14 by metal sleeve 11.
As shown in FIG. 1, magnetic flux masking shield 31 exhibits an arc-shaped curved surface that mainly covers the upper half of induction coil 18, and is advanced and withdrawn with respect to the gap at either end of coil assembly 10 and holder 12 by means of displacement mechanism 40. Displacement mechanism 40 has a wire 33 linked to magnetic flux masking shield 31, a pair of pulleys 36 on which wire 33 is suspended, and a motor 34 that rotates one of the pulleys 36.
When the size of recording material 14 is the maximum size, magnetic flux masking shield 31 is moved by means of displacement mechanism 40 so as to be withdrawn into the position shown by the solid line in FIG. 1. On the other hand, when the size of recording material 14 is small, magnetic flux masking shield 31 is moved so as to advance into the position shown by the dot-dot-dash line in FIG. 1. By this means, magnetic flux reaching a paper non-passage area of metal sleeve 11 from induction coil 18 is masked, and an excessive rise in temperature of the paper non-passage area is suppressed.
FIG. 2 is a characteristic graph showing the characteristic of surface temperature with respect to axial direction position of a heating roller in a fixing apparatus disclosed in Patent Document 2. In this fixing apparatus, when the heat-fixing of small-size paper is performed repeatedly, the surface temperature distribution of the heating roller shows a considerable rise in paper non-passage areas at either side of the paper passage area immediately after passage of the aforementioned small-size paper, as shown by the solid line in FIG. 2.
Thus, in this fixing apparatus, in the above-described situation, the heating roller is cooled by alternating between above-described rotational cooling and above-described static cooling. That is to say, the surface temperature of the heating roller is lowered by the rotational cooling as shown by the dot-dash line in FIG. 2, and the surface temperature of the heating roller is made uniform by the static cooling as shown by the dot-dot-dash line in FIG. 2.
Patent Document 1: Unexamined Japanese Patent Publication No. HEI 10-74009
Patent Document 2: Unexamined Japanese Patent Publication No. 2003-173103